Extraordinary optoconductance (EOC) is a recent example of a class of geometry-driven interfacial “EXX” phenomena exhibited by metal-semiconductor hybrid (MSH) structures, wherein:                E=extraordinary; and        XX=a type of interfacial phenomena such as magnetoresistance (MR), piezoconductance (PC), optoconductance (OC) and electroconductance (EC).See the above-referenced and incorporated related patent applications; see also Wieland et al., “Extraordinary optoconductance in metal-semiconductor hybrid structures”, Appl. Phys. Lett. 88, 052105 (2006); Solin et al., “Enhanced Room-Temperature Geometric Magnetoresistance in Inhomogeneous Narrow-Gap Semiconductors”, Science 289, 1530 (2000); Rowe et al., “Enhanced Room-Temperature Piezoconductance of Metal-Semiconductor Hybrid Structures”, Appl. Phys. Lett. 83, 1160 (2003); Rowe et al., “Giant Room-Temperature Piezoresistance in a Metal-Silicon Hybrid Structure”, Phys. Rev. Lett. 100, 145501 (2008); and Wang et al., “Extraordinary electroconductance in metal-semiconductor hybrid structures”, Appl. Phys. Lett. 92, 262106 (2008), the entire disclosures of each of which is incorporated herein by reference. The term “EXX sensor” thus refers to a class of MSH devices having a semiconductor/metal interface whose response to a specific type of perturbation produces an extraordinary interfacial effect XX or an extraordinary bulk effect XX. The interfacial or bulk effect XX is said to be “extraordinary” as that would term would be understood in the art to mean a many-fold increase in sensitivity relative to that achieved with a macroscopic device for the same perturbation.        
For example, the above-referenced and incorporated patent applications describe an EOC device such as the one depicted in FIG. 1. FIG. 1 illustrates an exemplary EOC sensor 100, which is an MSH device having a semiconductor portion 102 and a metal shunt portion 104 which are disposed on a substrate 106. Together, the semiconductor portion 102 and the metal shunt portion 104 define a semiconductor/metal interface 108. As shown in FIG. 1, with the EOC sensor of FIG. 1, the semiconductor portion 102 and the metal shunt portion 104 are substantially co-planar. Furthermore, the semiconductor portion 102 and the metal shunt portion 104 lie in a substantially parallel plane as the substrate 106. Further still, it can be seen that the semiconductor/metal interface 108 is substantially perpendicular to the plane of substrate 106. This architecture can be referred to as an externally shunted van der Pauw (vdP) plate.
In operation, the EOC device 100 is perturbed with light 102 from a light perturbation source 120 (which can be any source of light emissions, including but not limited to a laser emitting device, cells with fluorescent emissions (such as would be emitted with the introduction of a fluorine-based contrast agent), etc.). The light 122 which impacts the light exposed surfaces of the semiconductor portion 102 and metal shunt portion 104 results in the semiconductor-metal interface 108 behaving as an Ohmic (or linear) interface and produces a measurable voltage via an EOC effect. Experimentation with EOC devices 100 with the general architecture shown in FIG. 1 wherein the EOC devices have length (y-axis) and width (x-axis) dimensions of 500 nm or more have shown such EOC devices to exhibit an effective resistance that decreases with increased illumination intensity.
Another example of an EXX sensor described in the above-referenced and incorporated patent applications is the EEC device 200 depicted in FIG. 2. The EEC sensor 200 of FIG. 2 is also an MSH device comprising a semiconductor portion 202 and a metal shunt portion 204. With the EEC device 200 of FIG. 2, the metal shunt portion 204 is disposed on a surface of the semiconductor portion 202, and the semiconductor portion 202 is disposed on a surface of substrate 206 such that the semiconductor portion 202 is sandwiched between the metal shunt portion 204 and the substrate 206. As shown in FIG. 2, the metal shunt portion 204, the semiconductor portion 202, and the substrate portion 206 preferably lie in substantially parallel planes. Together, the contact between the metal shunt portion 204 and the semiconductor portion 206 define a semiconductor/metal interface 208. Thus, unlike the EOC device 100 of FIG. 1, the plane of the semiconductor/metal interface 208 of EEC device 200 is substantially parallel with the plane of the metal shunt/semiconductor/substrate. In operation, an electric charge created by an external electric field perturbs the EEC device to produce a voltage response via an EEC effect.
Panel (a) of FIG. 3 shows a perspective schematic view of an exemplary EEC van der Pauw structure upon which experiments have been conducted. The metal shunt portion (with a 50 μm radius) is concentric and in direct contact with the semiconductor portion (a GaAs mesa with a radius of 100 μm). Four leads are deposited on the periphery of the mesa surface and lead 5 is directly connected to the metal shunt. Panel (b) of FIG. 3 shows an SEM image of such a device but without the metal shunt on top. Panel (c) of FIG. 3 shows a cross-sectional view of the EEC's multilayer structure. As can be seen, the metal shunt comprises two 50 nm thick metal thin films Ti and Au/Ge. The inventors note that multiple layers of metal can be used in the metal shunt portion 204 to promote adhesion. For example, Ti sticks well to GaAs, but Au/Ge is a better conductor. Thus, by using both Ti and Au/Ge in the metal shunt, the device can leverage both advantageous properties. Also, the semiconductor portion comprises an Si-doped GaAs epitaxial layer (200 nm thick) and an undoped GaAs epitaxial layer (0.8 μm thick). The undoped semiconductor layer was positioned on a substrate portion that comprises a semi-insulating GaAs substrate (350 μm thick). A pair of parallel plates (0.2 μm Au/Ge plates) is incorporated to apply an external electric field with a 1 μm Si3N4 dielectric between the top plate and the metal shunt. In experimenting with such an EEC device, the inventors serendipitously discovered that such an EEC device exhibited a very strong sensitivity to light. Further still, when reducing the scale of this device to a nanoscale, the inventors discovered not only a strong sensitivity to light but also a fundamentally different type of sensitivity to light than observed for the larger macroscopic EEC device. That is, the inventors discovered that a device having the architecture of the EEC devices of FIGS. 2 and 3 could also serve as an EOC device that produces an extraordinary response to light perturbations. Moreover, when the device dimensions were reduced to the nanoscale, the inventors unexpectedly discovered that the device exhibits an inverse and much larger EOC response (an “I-EOC” response) in which the effective resistance of the device increases with increasing illumination intensity (in contrast to observations in connection with devices having a macroscopic scale where the effective resistance decreased with increasing illumination intensity). As a result of experimentation and analysis, the inventors believe that by reducing the spacing between the Ohmic leads and the metal shunt to a value less than a maximum of around 10 times the mean free path of the carriers in the active semiconductor layer such an I-EOC effect becomes possible. The inventors further believe that reducing this spacing to values below 500 nm makes the I-EOC effect more pronounced. The inventors further believe that the spacing may become too little for an I-EOC effect when too many electrons begin to tunnel from the electrode to the shunt through the semiconductor. The inventors believe that this minimum threshold would be reached at a spacing of around 1-3 nm. Furthermore, the inventors discovered that this I-EOC effect is achieved in room temperature environments (i.e., around 300 K or 27° C.).
As used herein, “nanoscale” refers to dimensions of length, width (or diameter), and thickness for the semiconductor and metal portions of the EXX sensor that are not greater than approximately 1000 nanometers in at least one dimension. As used herein, “microscale” refers to dimensions of length, width (or diameter), and thickness for the semiconductor and metal portions of the EXX sensor that are not greater than approximately 1000 micrometers in at least one dimension.
The room temperature I-EOC effect exhibited by embodiments of the present invention is believed to be fundamentally distinct from the negative photoconductivity seen in laterally macroscopic semiconductor heterostructures or in functionalized nanoparticle films (see Nakanishi et al., “Photoconductance and inverse photoconductance in films of functionalized metal nanoparticles”, Nature 460, 371 (2009)). For example, the inventors believe that the I-EOC effect exhibited by embodiments of the invention is not dependent on trap states but rather on a transition from ballistic to diffusive transport, as explained below. Moreover, although much work on nanowire-based nanophotonic devices have been reported, the inventors believe that such nanowire-based nanophotonic devices are not currently compatible with Very/Ultra Large Scale Integration (VLSI/ULSI) fabrication methods. (See Law et al., “Nanoribbon Waveguides for Subwavelength Photonics Integration”, Science 305, 1269 (2004)). By contrast, the I-EOC devices described herein are VLSI-compatible, individually-addressable, and exhibit significant sensitivity in the visible light spectrum. As such, the inventors believe that the I-EOC nanosensors described herein have beneficial applications in a wide variety of nanophotonic applications, ranging from medical imaging for diagnostics to information technology and communication. (See Law et al., “Nanoribbon Waveguides for Subwavelength Photonics Integration”, Science 305, 1269 (2004)). For example, the EOC and I-EOC sensors described herein can be employed in applications including, but not limited to, contact imaging, astronomical detection and observation, video cameras, still cameras, cancer detection, blood analysis, nanoparticle diffusion and size studies in industrial processes, position-sensitive detection and optical information storage and detection. When used in connection with contact imaging, embodiments of the invention can be employed as described in the above-referenced and incorporated Ser. No. 12/375,861 application. For example, an object can be brought into proximity or contact with a dense array of EOC or I-EOC sensors described herein, light can be passed through the object, and voltage readings from the sensor leads can be used to generate pixels for images of the object. With applications to astronomical detection/observation, video cameras and still cameras, the EOC and I-EOC sensors disclosed herein can be used as an optical sensor akin to CCD devices (wherein each EOC/I-EOC sensor would effectively serve as a pixel sensor). For applications involving cancer analysis and detection, cancer analysis/detection might be accomplished by detecting fluorophores or other light emitting agents (bioluminescence, etc.) that bind to pathological molecules or are expressed by transformed or transfected cells, either in vivo or in vitro. For applications involving nanoparticle diffusion and size studies, the inventors note that a medium that contains the subject nanoparticles can be brought into proximity or contact with an array of EOC/I-EOC sensors described herein, and light can be passed through the medium to impact the array. The voltage readouts on the sensor leads can then be monitored for intensity distribution as a function of time. For position-sensitive detection applications, for example, the EOC/I-EOC sensors described herein can be used in a similar manner as described for nanoparticle diffusion, but a moving (micro or nano) flag (such as a hole attached to or in the object whose position it to be detected) is included. In an application relating to information storage, a single I-EOC sensor (or small number of such sensors) could be placed on a moving arm of an optical information storage system to detect reflected light from an optical disc.
Therefore, in accordance with an aspect of an embodiment of the present invention, the inventors herein disclose a method that comprises perturbing a nanoscale MSH device with light to produce an EOC effect. Preferably, this EOC effect is an I-EOC effect. Furthermore, the MSH device may comprise a semiconductor material, a metal shunt located on a surface of the semiconductor material, thereby defining a semiconductor/metal interface, wherein a portion of the semiconductor material surface is not covered by the metal shunt, wherein the semiconductor material and the metal shunt are in substantially parallel planes but are not co-planar, and wherein the semiconductor/metal interface is configured to exhibit a change in resistance in response to the light perturbation.
In accordance with another aspect of an embodiment of the present invention, the inventors herein disclose a method that comprises perturbing a nanoscale MSH device having a semiconductor/metal interface that defines a Schottky barrier (non-linear) interface with light to produce an I-EOC effect.
In accordance with yet another aspect of an embodiment of the present invention, the inventors herein disclose a method that comprises perturbing an MSH device with light to generate an EOC response, wherein carrier flow within the MSH device across the semiconductor-metal interface transitions from primarily a ballistic transport to a diffusive transport in response to the perturbation by the light.
Further still, in accordance with another aspect of an embodiment of the present invention, the inventors herein discloses a method that comprises perturbing an MSH device with both light and electric charge perturbations to produce an EEC and EOC response.
Furthermore, in accordance with yet another aspect of an embodiment of the present invention, the inventors herein disclose the combination of multiple I-EOC devices in an array such that an imaging device having pixels of nanoscale resolution is created. Such an array can be perturbed with electric charge and/or light to create images having a nanoscale resolution.
These and other features and advantages of the present invention will be described hereinafter to those having ordinary skill in the art.